For as long as the automobile has been with us there are many aspects of its design and maintenance that are still not well understood by the people (even engineers) who have to dealt with them. (One of the reasons why progress in this field has been so painfully slow.) Typical of these neglected factors is the importance of road wheel balance.
Equipment capable of properly balancing wheels is complex and expensive--consequently not commonly available. Even then, the principles are often not properly understood by the mechanics who use them so that the results are compromised.
It is no wonder that the average vehicle owner has become accustomed to an unnecessary level of roughness in the road performance of his car which he accepts as normal --even to the point where it is dangerous to handling. These conditions also cause accelerated wear on tires and vehicle suspensions.
Driving along a motorway it is surprising the number of cars where one observes one or more wheels whose vibration is noticeably visible. Of course, detrimental performance occurs far below the point of external visibility.
In fact, wheels should be balanced to a high degree of precision and then rebalanced after a significant amount of tire wear has occurred, say 5 to 10 thousand miles or, of course, whenever a tire has been remounted on a wheel for any reason, such as repair of a puncture.
Of all the components of the running gear of an automobile, wheels and tires are the least precise. Due to the manner of their manufacture they are dimensionally relatively crude devices. The assembled combination can quite normally give lateral and radial run out of as much as 1/16 inch (permitted by industry standards) and even 1/8 inch is not uncommon. For this and other contributory reasons intrinsic to the manufacturing methods employed, the possibility of very large mass imbalances must be expected and, indeed, do occur in practice. Consequently, the wheel/tire combination must be properly balanced, laterally and radially, before mounting on the vehicle and putting it in use.
To be properly balanced, a wheel must be in a state of equilibrium in two respects:
1) The distribution of the mass of the wheel must be perfectly concentric about its axis of rotation. (Radial balance--sometimes referred to as "static" balance.) PA1 2) The rotational plane of the mass must be exactly perpendicular to the axis of the rotation. (Axial balance--sometimes referred to as "dynamic" balance.) PA1 1) Bubble balancers; PA1 2) "On the vehicle" spin balancers; PA1 3) Wheel spin balancers. PA1 Meredith U.S. Pat. No. 2,442,308 PA1 Silver U.S. Pat. No. 3,077,781 PA1 Trimble U.S. Pat. No. 3,147,624 PA1 Frank et al. U.S. Pat. No. 3,812,725 PA1 Finch et al. U.S. Pat. No. 3,991,620 PA1 Ito U.S. Pat. No. 4,011,761 PA1 Harant U.S. Pat. No. 4,149,416 PA1 Kogler et al. U.S. Pat. No. 4,173,146
The effect of radial imbalance is to cause the wheel to vibrate vertically, or bounce as it rotates. Axial imbalance is felt as angular steering wheel vibration--sometimes referred to on the front wheels as "shimmy".
Both of these conditions, in addition to being unpleasant for the driver and passengers, reduce the effectiveness of tire adhesion to the road. Therefore, under marginal conditions of rain, snow and ice at road speed they can also be very dangerous.
Because of the precision with which the balancing must be carried out, it is a difficult problem to solve in a practicable, manageable way. It also requires precision manufacture of the mounting and locating surfaces on the vehicle--something which the more advanced and knowledgable automobile manufacturers are now doing. For example, 0.005 inch radial offset from the true center is approximately equal to a 1/2 oz. weight at the wheel rim. This degree of imbalance is perceivable on most cars. Therefore, a maximum combined error which includes the tolerances on the vehicle wheel mounting and the balancing machine should be substantially less than 0.0025 inch.
Put another way, it must be possible to reliably detect, measure and correct a small imbalance in the radial plane acting at a minimal radius of 7 inches to an accuracy of 0.03% of the wheel mass (a static torque of about 2 inch ounces). The axial balance accuracy requirement can be similarly stated as a 2 to 3 inch ounce static torque correction (dynamically the magnitude of this torque couple is, of course, a function of wheel rotation velocity).
All wheel balancing methods attempt to sense the amount of mass imbalance and to compensate by the installation of properly located increments of weight of the required amount fastened to the wheel rim.
There are three general classes of equipment in common use for this purpose:
Only machines in category (3) are capable of satisfactory results.
Bubble balancers are widely used because they are inexpensive. However, they are very inaccurate and hopelessly confuse the effects of the two types of mass imbalance. They can, therefore, provide only a very coarse correction which is normally far outside the requirements for acceptable road performance.
Machines in category (2) attempt to balance the wheels while installed on the vehicle. One of the justifications for this technique stems from the fact that some manufacturers do not adequately control their wheel locating dimensions. However, it can be readily shown that it is theoretically impossible to separately detect and measure the two components of imbalance using this method. Nevertheless, in practice it is possible, with sufficient patience, to achieve compromise conditions which represent some improvement. Of course, the wheels so treated must not be removed and remounted in another location. This type of balancer must also be considered to be unsatisfactory.
There are a number of different types of category (3) machines. Basically, they all operate on theoretically sound principles. In one way or the other they properly sense the two components of imbalance and provide for their appropriate correction. However, these machines are large, heavy, complex and expensive. Consequently, they are out of the financial reach of the largest sectors of the automobile service industry. Moreover, some, by their design, are susceptible to large operator errors so that their inherent accuracy is often not realized in practice. But, when properly used, the better machines in this category represent the standard by which other methods should be compared. However, most such machines require that the wheel be driven at relatively high rotational speeds to generate out of balance forces which are measured to provide an indication of imbalance weight and position (e.g., see U.S. Pat. No. 3,910,121).
In my U.S. Pat. No. 3,847,025 I disclose a dynamic wheel balancing method and apparatus in which the wheel is mounted in a pivoted cradle mechanically coupled to a maximum displacement indicator. The measurements of wheel imbalance and location were all mechanical and locating of the placement points for the weights was by trial and error.
Jackson U.S. Pat. No. 4,007,642 discloses a system using piezoelectric load cells to detect imbalance of a rotating wheel; Hale et al. U.S. Pat. No. 3,527,103 also drives a wheel at relatively high rates of speed to detect radial deviation to locate tire imperfections; Newkirk U.S. Pat. No. 1,557,268 discloses a pivoted cradle having an axle for rotating a body about an axis transverse to the cradle pivots and a long arm to an indicating scale indicates the amplitude of imbalance.
Other prior art is as follows: